Q.: We placed an industrial building floor using shrinkage-compensating concrete. This type of concrete was chosen so there would not be as many joints to maintain. Though the floor has very few joints, each joint has opened to a width of nearly 1/2 inch, making joint maintenance very difficult. The floor consists of 1,000-square-foot panels, placed one after another in checkerboard fashion. Isn't this type of concrete supposed to expand, not shrink?
A.: Most likely, expansion is the cause of your problem. To understand why, you must learn how the timing of expansion and contraction affects shrinkage-compensating concrete. That's right, contraction. Shrinkage-compensating concrete expands during the curing process, usually for the first few days to one week after placement. After expansion, the drying shrinkage of shrinkage-compensating concrete is similar to that of portland-cement concrete.
Shrinkage-compensating concrete is designed to increase in volume after setting and during hardening. If this expansion is restrained by reinforcement, it will cause tension in the reinforcement and compression in the concrete. When drying shrinkage occurs, it merely reduces the overall compression stresses in the slab, instead of causing a tensile stress that leads to cracking. A similar reaction occurs in prestressed concrete.
For slabs on grade, ACI 223, "Standard Practice for the Use of Shrinkage-compensating Concrete," does not recommend the checkerboard placement sequence unless a compressible joint material is placed between the slabs prior to placement. The placement sequence must allow the concrete to expand.
In a checkerboard sequence, the floor placements bordered on all sides by previously placed panels have no room for expansion. These placements are prone to cracking when drying shrinkage occurs, since expansion has been restrained. Drying shrinkage of shrinkage-compensating concrete is affected by the same factors as portland-cement concrete, such as water content, aggregate type, and cement content.